If you want your extra leisure/auxiliary battery to get fully charged you have to have a 12V to 12V charger.
If your starter battery and your extra battery is LiFePo4 then you most certainly have to have a charger to save your alternator.
The resistant in a Lithium battery is a lot less than a Lead acid and will withdraw excessive amount of power from the alternator
I have found two solutions for Dc to Dc charger here below
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